If people are standing or sitting on higher stools then the viewing angle should be ok so I would go for flat if you can to get more people to use it at once.
In order for it (LCD) to be transparent you will need to remove the backlight whic can be quite trick on the larger moniters and light it some other way as well as have the cammera far enough back to capture the whole lot.
The cammera is important as others have said with the lag, the wiimote one is 30fps i beleive and this can be had in other quality cammeras to so you just need one with an alright framerate and resolution.
The number of points tracked is dependant on the software that you use for tracking and the resolution of the cammera along with the physical size of the table each of these helps determine how many clearly defined points are able to be interprated by the system.

